Bottom bracket loose on a fixed gear.
 

I have a fixed gear bike and the bottom bracket loosens after some use even after tightening it. I’ve used the BB for just a month but it’s a fairly cheap one and I’m wondering if that’s normal and if a better BB would fix my problem.

What kind of BB should I use to solve that? My budget is tight and I can’t afford changing the whole crankset, but I’ll save for a quality BB if that solves my problem. I have Miche Primato cranks.

My local bike mechanic suggested me to save for something like the omniums or some second hand dura ace but I hope that isn’t needed.

No it’s not normal. BBs should be “set it and forget it”. Did you use a torque wrench to install the bb? These usually need significant grunt to install correctly. What can you tell us about the BB itself? Brand, type, threading etc. is one side in particular loosening?

The BB unscrews with the use and goes out to the left side. I think if I skid it happens faster but it’s hard to tell.

I don’t really know anything about the BB, just that it costed me around 20€ Installation included. It was installed with a torque wrench and the first time it loosened they tightened it again in the workshop with the wrench. Since then I’ve been doing so myself with a screwdriver and a hammer until I can get a permanent solution.

The type of BB is important. For example, is the inside of the BB threaded or were the bearing outers / cartridge a press fit in the BB? On my bikes I have to press the bearing outers in, then fit the bearing/crank, and then there is a rubbishy slotted nut that holds it all together on one side. Leaving the bike alone somewhere damp for a month so that the nut begins to rust onto the crankpin thread can help, or so I am told ;)

It is important whether the bottom bracket bearing has an adjustable (non-drive side) cup and lockring, or whether it is of cartridge type.

Based on what you've said, this sounds like the older-style adjustable type. If it has caged bearings within the two cups, you might want to uncage the bearings and re-install. This method has solved a similar problem for me over the years. Also, the shop may have inverted the cages, which would be a sad thing to discover.

Proper torque to the lockring is important, too.

If you have the cartridge type, I'm at a loss to advise you.

It's a square tape BB and after doing some reasearch I can tell you that the non drive cup is what unscrews and exits the shell. Any ideas why is this happening?

Thaks all of you for replying I'm trying to figure all this out and your guidance helped a lot.

I'm not a native english speaker and I misunderstood what a torque wrench was. No, it wasn't tightened with a torque wrench. I guess that might be it and it just needed a more precise installation.

If it wasn't tightened with a torque wrench it a very good chance it wasn't tightened nearly enough. The recommended torque is in the range of 40 to 45 Nm and that's MUCH tighter than you think and much tighter than a small wrench will allow.

After dealing with the BB issue the OP should revisit the chain tension. We see fixed gear bikes with way too tight a chain quite often. It is pretty impressive in how much force a too tight chain places on the drive side BB bearing (and it's fitting within the shell or cup) and the rear hub's drive side bearings.

So what's the correct tension a chain (on a fixed or single speed) should be set at? Trick question because the answer is zero tension. Of course with some set ups the cog and ring are not completely round thus as one pedals the chain tension varies. We set the tension at the tightest point with JUST a tad of slop so when the loosest point is experienced the chance of chain derailment is the least. Andy

Thanks for the tip. Loosening the chain tension is the first thing I tried after tightening the BB. It didn't fixed the issue but it slowered the rate at which the BB unscrews. I think the tension now is pretty reasonable, I set the lightest tension it is pleasant to ride.

Several thoughts - I not claiming I know the answer for you. I always set my chain to never go tight and like to see around a 1/2" (1.3cm) of slack. Now, unless the crankset, bottom bracket and hub are of high quality, the chainrings and cogs don't spin perfect circles. This means the chain goes tight and loose as you pedal. The worse this is, the harder it becomes to dial in the "perfect" chain slack. (Cranksets made of road use with derailleurs are often the worst offenders because there is no incentive to make them "round". With a rear derailleur providing tension, the additional coast of manufacture gains nothing - until you decide to fo fix gear or single speed.)

Are you greasing the threads of the left side cup? (This is a cup and lockring with notches isn't it? The special tool that fits those notches will be a great asset. (Either a VAR/TA style with a ring and the appropriate prongs or a more general Park style spanner. A pin wrench to hold the cup also is a benefit. There are enough different styles that I have no idea what your setup needs but the correct tools make life far easier.)

There are those who are fans of using one of the LockTights on the threads. Phil Wood in fact says to do so with their adjustable bottom brackets. I do as Phil Wood says on their BBs but use only plenty of grease and or Teflon plumbers tape on my other BBs. (I don't always remember to use the plumbers tape, but when I do, I never regret it.)

Now, I don't skid stop. My knees wouldn't last much longer than a tire. Plus, I love the ride of really good tires and skidding gets really expensive fast.

Edit: "lightest tension that is pleasant to ride" meaning no "kick-back when you reverse pedaling direction? Too tight! Use the kick-back to teach yourself to pedal evenly. Go to slow down and feel that one nice "click" . That kick-back and learning to avoid it is one of the best smooth pedaling tools there is. (Go to the velodrome and you will see a whole lot of really skilled riders and very expensive fix gears all with a ton of chain slack. You want a touch less slack because you have road bumps and they do not but you want to be as close to what they do as you can while guaranteeing your chain stays on.

In the auto world this is sometimes called rubber banding. Where the tolerances through the system will wind up and play out depending of which way the system is being loaded. I dislike it in a car but it is the result of proper chain set up on a bike. Andy

On my coaster brakes I have some slack for two reasons. The first is to avoid brake snatch on coasting (not relevant to a fixie), and the second because of the kick-back. Hiding the kick-back through chain tension is not the same as removing the forces that create it. On loose surfaces the tire can slide, but on tarmac those forces generated as the tire bumps down the road will be shared by your legs, your ass and your BB.